Factor VIII is a plasma glycoprotein that is essential for normal hemostasis. Defective or absent factor VIII causes hemophilia A, a life-threatening bleeding disorder. Because the gene for factor VIII resides on the X chromosome, the disease occurs almost exclusively in males, the sons of mothers with one defective factor VIII gene.
In the absence of therapy, hemophilia A is usually fatal prior to the age of reproduction. However, intravenous infusion of factor VIII alleviates the bleeding risk. In developed countries patients are routinely treated with pharmaceutical factor VIII so that the life-span for hemophilia A patients now approaches the life span of unaffected patients.